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Doppler Ultrasound is really just normal Ultrasound signals that have been processed in a different way.
Instead of the scanner detecting and measuring the size, or amplitude, of a returning echo; in Doppler ultrasound, the change in frequency of returning echoes is used to determine how a body is in motion.
To simplify, consider an ambulance passing you by. As the vehicle travels towards and then away from you, you will hear the siren's pitch changes. First the pitch becomes higher and then lower.
Originally discovered by the Austrian mathematician and physicist, Christian Doppler (1803-53), this change in pitch results from a shift in the frequency of the sound waves, and is known as the Doppler Shift.
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As the ambulance approaches you, the sound waves from its siren are compressed towards the observer. The intervals between successive waves diminish, which is detected as an increase in the frequency or pitch. As the ambulance moves away, the sound waves are stretched out, causing the siren's frequency to decrease. You can thus determine if the ambulance is coming nearer or speeding away just by its sound and by measuring the rate of change of the pitch, you could also estimate the ambulance's speed.
So, the Doppler effect is a change in the frequency of a wave, resulting from motion of the wave source or receiver, or in the case of a reflected wave, motion of the reflector.
In medical imaging, Doppler Ultrasound is used to detect and measure the blood flow, and the major reflector is the red blood cell (erythrocyte).
The strength of the Doppler shift is dependent on the transmitted frequency, the velocity of moving blood, and the angle between the sound beam and direction of moving blood, as expressed in the Doppler equation
Df = 2 f v cos θ
c
where Df is the Doppler shift frequency (the difference between transmitted and received frequencies), f is the transmitted frequency, v is the blood velocity, c is the speed of sound, and q is the angle between the sound beam and the direction of moving blood. The equation can be rearranged to solve for blood velocity, and this is the value calculated by the Doppler US machine:
V = Df c
2 f cos θ
The Ultrasound scanner can not detect whether the blood is arterial or venous but displays movement away from the transducer in one colour and movement away in another, with in the 2D Image display, with relative velocities indicated by shades of colour. The information is also often shown in graphical formal, usually showing frequency shift against time.
Modern processing and imaging techniques give the Clinical Vascular Scientist access to a vast range of additional and detailed information with regard the blood flow, much enhancing the diagnostic process.
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